In a first aspect the present invention relates to a hearing aid comprising a behind-the-ear unit, an in-the-ear unit and an interconnecting element arranged therebetween, wherein the in-the-ear unit comprises an inflatable element which when inserted into an auditory canal of a user may be inflated to abut an inner surface of the auditory canal so as to retain the inflatable element in the auditory canal, wherein the interconnecting element defines at least one conduit which is fluidly connected to the inflatable element and to a pump arrangement which is provided in and/or on the behind-the-ear unit; wherein the pump arrangement comprises:                a pump chamber defining an inlet and an outlet;        a unidirectional inlet valve provided in the inlet, the inlet valve allowing downstream flow of air into the pump chamber and limiting upstream flow of air out of the pump chamber;        a unidirectional first outlet valve provided in the outlet, the first outlet valve allowing downstream flow of air out of the pump chamber and limiting upstream flow of air into the pump chamber; and        a unidirectional second outlet valve provided downstream relative to the first outlet valve, the second outlet valve allowing downstream flow of air and limiting upstream flow of air;        wherein a valve leakage of the second outlet valve is lower than the valve leakage of the first outlet valve, the valve leakage being defined as the flow rate of air leaking/flowing through the valve in the upstream direction, when the valve is closed.        
The outlet of the pump arrangement comprises two valves which are arranged in series. The first outlet valve is less tight than the second outlet valve. The provision of the second tighter valve prevents or limits backflow of air from the inflatable element and back into the pump chamber. At the same time, the first valve may be a valve which opens more rapidly than the second valve. This way, it may be ensured that the overpressure provided by the pump (which is arranged downstream the first outlet valve) may be quickly transferred downstream the first outlet valve and subsequently gradually through the second outlet valve as the second outlet valve opens more slowly. Moreover, as the second outlet valve is more tight, any air which passes through the second valve is less likely to flow back upstream.